The oxidation of hydrocarbons is an important reaction in industrial organic chemistry. For example, the oxidation of cyclohexane is used commercially to produce cyclohexanol and cyclohexanone, which are important precursors in the production of nylon, whereas oxidation of alkyl-substituted aromatic hydrocarbons is used to produce phenol, a precursor in the production of polycarbonates and epoxy resins.
Oxidation of hydrocarbons can be conducted using well-known oxidizing agents, such as KMnO4, CrO3, and HNO3. However, these oxidizing agents have the disadvantage of being relatively expensive, and moreover their use is accompanied by the production of unwanted coupling products which can cause disposal problems and ecological pollution.
Thus, oxidizing agents based on peroxides or N2O have been used. The cheapest oxidizing agent, however, is molecular oxygen, either in pure form or as atmospheric oxygen. However, oxygen itself is usually unsuitable for oxidizing hydrocarbons, since the reactivity of the O2 molecule, which occurs in the energetically favorable triplet form, is not sufficient. As such, various catalysts have been developed to mediate the oxidation of an organic substrate using O2.
For example, International Publication No. WO2009/058527A1 discloses the use of a cycloimide as an effective catalyst in the oxidation of hydrocarbons to produce a hydroperoxide thereof. The use of the cycloimide catalyst, especially N-hydroxyphthalimide (NHPI), was found to be particularly conducive to a high oxidation conversion of the hydrocarbon and the selectivity of certain desired hydroperoxides. This reference also discloses the reclamation and recycle of the cycloimide catalyst from the reaction mixture.
The oxidation reaction of hydrocarbons by O2 is typically conducted in a bubble column reactor to which a liquid mixture comprising the hydrocarbon to be oxidized is fed, through which a stream of O2-containing gas, such as air or pure O2, is bubbled. The stream of gas is desirably introduced into the liquid mixture in proximity to the bottom, forming bubbles traveling through the column so that there is sufficient contact between the O2 and the hydrocarbon molecules. While the O2 bubbles travel upwards, they tend to agitate the liquid, resulting in better mixing of the whole reaction system. On the other hand, it was also observed that, in certain reaction systems, such as those using pure hydrocarbons as the reactants, at and/or close to the beginning of the oxidation process, a substantial amount of foam can be generated atop the liquid mixture by the agitation. The presence of a thick layer of foam can be detrimental to the oxidation operation for various reasons.
Hence, there is a need for an improved oxidation process for hydrocarbons with suppressed foaming at least at and/or close to the beginning of the oxidation operation.